Complementary metal-oxide-semiconductor (CMOS) technology is the dominant semiconductor technology used for the manufacture of ultra-large scale integrated (ULSI) circuits today. Current CMOS transistors typically utilize polysilicon as the gate electrode for both NMOS and PMOS transistors, wherein the polysilicon is doped with an N-type dopant to form NMOS transistors and is doped with a P-type dopant to form PMOS transistors. Polysilicon gates, however, often exhibit gate depletion problems.
As a result, CMOS transistors with metal gates have been attempted. Furthermore, because PMOS and NMOS function differently, it is desirable to fabricate PMOS and NMOS transistors having gates of different work functions. Generally, this is obtained by using different metal gates on the PMOS and NMOS transistors. One method of fabricating PMOS and NMOS transistors is by depositing a first metal layer on both the NMOS and PMOS gates and etching the metal layer from one of them. The etching process, however, frequently damages the gate dielectric layer, thereby affecting the performance of the transistor.
Another method that has been attempted is to deposit a metal layer for both the NMOS and PMOS transistors, but then performing an ion implant to alter the work function of either the NMOS or PMOS transistor. This method, however, is difficult to optimize the work function of both the NMOS and PMOS transistors.
Therefore, there is a need for an apparatus, and a method of manufacture, with metal gates having a first work function for a PMOS device and a second work function for an NMOS device that may be fabricated in a cost-effective manner.